The present invention pertains to a sealing station in a thermoforming machine for sealing a cover sheet to a bottom sheet.
Thermoforming machines are known as a component of blister packaging machines. They comprise a forming station, in which a plurality of pocket-like depressions or wells is formed in a bottom sheet, which can consist, for example, of plastic or aluminum or a combination of the two. In a downstream filling station, a product, such as a pharmaceutical tablet, is laid into each of these pockets. After the pockets have been filled with the product, the bottom sheet is sent to a sealing station. Just in front of the sealing station or inside it, a cover sheet is supplied and laid on top of the bottom sheet. Through the action of heat inside the sealing station, the cover sheet is sealed tightly to the bottom sheet, as a result of which the product is sealed in the pockets or pocket-like depressions.
A sealing station generally consists of a first or upper mold, which is heated, for example, by heating cartridges, and a second or lower mold, which represents the counterpart to the first mold and comprises cavities for holding the bottom sheet with its pocket-like depressions. Because the facing surfaces of the cover sheet and bottom sheet are pressed against each other and thus are at least partially welded together, a great deal of heat is transferred during the sealing operation from the upper mold to the cover sheet and also to the bottom sheet.
Although heat is transferred only during the actual sealing time, the layers of high thermal conductivity present within the composite sheet structure, the materials of which can comprise aluminum, lead to the result that the heating of the composite sheet structure consisting of the cover sheet and bottom sheet affects the product lying in the cavities and sealed in on all sides by the composite sheet structure. In the case of certain products, especially sensitive pharmaceutical capsules, for example, a maximum temperature may not be exceeded at any time during the packaging process, because otherwise the product could suffer damage, in the worst case even decomposition could occur. For this reason the second or lower mold must be kept at a predetermined cooling temperature which may not be exceeded. At the same time, the composite sheet structure must be cooled down immediately after the sealing operation.
For this purpose, the lower molds of thermoforming machines usually comprise a water cooling system, wherein, in the lower mold, channels filled with cooling water are present, which ensure in particular that the temperature of the lower mold does not increase through contact with the heated upper mold during the sealing time but instead remains below a certain temperature threshold.
U.S. Pat. No. 3,628,306 discloses a device for packaging articles between two heat-sealable plastic sheets. During the sealing operation, the composite sheet structure is cut along the product-free sealing area by a knife held perpendicular to the composite sheet structure. To cool the composite sheet structure in this area so that uniform cutting is ensured, air slots are provided in the lower mold. This device, however, does not cool the product enclosed by the composite sheet structure but rather only the sections of the sheet structure lying in the sealing area. A device of this type is therefore unsuitable for especially heat-sensitive products.
DE 34 04 451 A1 describes a packaging machine with an evacuation and sealing station for welding monofilms by means of a continuously heatable welding element. So that the monofilm is not destroyed by contact with the continuously heated welding element, a control unit is provided, which is designed so that it generates a pressure difference in the interior of the chamber between the pressure above the upper film and the pressure below it, so that the upper film always lies taut on the product and on the lower film during and also after contact with the welding element. Only after a cooling phase is the pressure difference removed, so that the package with the film, which has now been stabilized again, can be lifted and sent further along. In the lower mold of the evacuation and sealing station, channels are formed, through which a negative pressure is generated so that the film and the product rest directly on the top surface of the lower mold.